In recent years, more stringent regulations and greater appreciation of the importance of environmental preservation have lead to increased demands for materials that, as far as possible, contain no toluene as a solvent, nor ketone-based solvents or ester-based solvents. Against this background, alcohol solvents are attracting considerable attention, and in particular, the effective utilization of bioethanol, which has minimal environmental impact, holds considerable promise. However, when acrylic polymers are synthesized using an alcohol solvent as the polymerization solvent, because chain transfer to the alcohol solvent is significant, it is difficult to increase the molecular weight of the polymer, and achieving satisfactory adhesive performance is problematic. Moreover, in the case of monomers having ethylenic unsaturated bonds such as vinyl acetate, acrylonitrile and acrylamide, which are suspected of being carcinogenic, safety concerns such as the effects on people have resulted in increasing demands for eliminating the use of such monomers in the production of acrylic polymers. If these types of monomers are not used, then the cohesion tends to deteriorate, and the adhesive strength tends to worsen, meaning achieving satisfactory adhesive performance is difficult. Achieving excellent adhesive performance requires a combination of a high-molecular weight polymer, and superior cohesion and wettability.
Known methods of polymerizing an acrylic polymer in an alcohol solvent include the methods disclosed in Patent Documents 1 to 3, which use a (meth)acrylate alkyl ester monomer and a monomer having two or more unsaturated double bonds.
However, in the examples disclosed in Patent Document 1, a batch method is used for the polymerization method, and because the amount of alcohol solvent used relative to the (meth)acrylate alkyl ester monomer is large, chain transfer to the alcohol solvent is considerable, and achieving a high-molecular weight polymer in a stable manner is problematic. Further, because an acrylate monomer is used as the monomer having two or more unsaturated double bonds, the monomer having two or more unsaturated double bonds exists until a late stage in the reaction process, and therefore localized increases in the molecular weight are more likely, and there is a tendency for gelling to occur.
Moreover, in Patent Documents 1, 2 and 4, because an isocyanate compound is used as a crosslinking agent, when the pressure-sensitive adhesive composition is stored for a long period of time, a reaction occurs between the isocyanate group and the alcohol solvent, resulting in a deterioration in the storage stability of the coating liquid, and as the reaction between the isocyanate group and the alcohol solvent progresses, other problems also arise, including a decrease in the amount of crosslinked structures, a deterioration (change) in the adhesive performance over time, and a deterioration in the releasability. Further, the adhesive strength to adherends cannot be said to be entirely satisfactory.
Furthermore, in Patent Document 3, besides the isocyanate compound, a titanium compound may also be used as the crosslinking agent, but the resulting adhesive strength, releasability, and heat-resistant holding power and the like are unsatisfactory, and the adhesive performance is not entirely satisfactory.
Further, Patent Document 4 states that excellent adhesive properties can be achieved by using an acrylic polymer containing a high-molecular weight component and a low-molecular weight component within a single polymer, in which the amount of the low-molecular weight component is greater than that of the high-molecular weight component. However, because this polymer lacks branched structures, the adhesive strength and wettability relative to adherends having poor adhesiveness such as polyolefins are inadequate, meaning the adhesive is not entirely satisfactory.